Thursday, March 11, 2010

GWWA in Honduras

The Golden-winged Warbler, according to BirdLife International (the official Red List authority for birds for IUCN), is currently listed as near-threatened. It has "declined rapidly in southern parts of its breeding range in recent years" (BirdLife International 2009).

Local declines of this early successional specialist "correlate with advancing succession and reforestation, and the invasive range expansion of Blue-winged Warbler Vermivora pinus. Other possible causes of population declines are loss of wintering habitat (especially forest edge and open woodland) through agricultural expansion and clearance for plantations, and nest parasitism by Brown-headed Cowbird Molothrus ater." (BirdLife International 2009).

In light of these declines, it is perhaps interesting - though of course largely anecdotal - to note that we saw quite a few Golden-winged Warblers last winter at our Golden-cheeked Warbler field sites. On this blog I even mentioned that in passing here, here and here.

We ended up seeing no fewer than 16 individual Golden-winged Warblers in Honduras last winter. We found the species at every field site we visited. If you've just tuned in, that means five different field sites in pine-oak forest throughout Honduras. At each site, we described at least five different insectivorous mixed species flocks, one per day, and indeed most of our GWWA sightings were of flock members, i.e. birds we found associating with other insectivorous species. The focus of our field work there was the Golden-cheeked Warbler, but we identified and counted all flock members, for a duration of about four hours per flock, over a total of 25 flocks.

At our first field site, in the forests around La Esperanza, we found 3 Golden-winged Warblers. At our next site, Cusuco, we saw 5. In La Botija, our third field site near the Nicaraguan border, we only found one individual. (Most of our locations there were fairly open situations with limited understory.) In La Tigra, not far from the Honduran capital Tegucigalpa, we encountered 4 individuals, while at our last field site, Monte Uyuca, we found 3.

So how does that compare to previous years? After all, this was the fourth year of visiting the same sites using the same protocol.

Well, as some of you will remember, the previous winter I worked not in Honduras but in Chiapas, Mexico. Another team collected the Honduran data. I have the Honduran site reports from that season, and it appears that at least one individual was seen at every site, for a minimum of 5 birds. I'll try to find out if they had more. The year before that, I was part of the Honduran field team and we certainly did not record the species at every site that year. In fact, we only found it in La Tigra (2) and Cusuco (4). The data of the very first year still exist of course but sadly I no longer have them; a computer crash separates me from that year's data. But if memory serves, we only saw 5 or 6 individuals that year.

So that number of 16 for last winter seems pretty good to me.

The majority of those 16 birds were males. In fact, we saw at least 10 males and at least 1 female; 5 birds were not reliably sexed.

With the very strong caveat that this represents only a tiny sample size, it's intriguing to note the skewed sex ratio, isn't it?

My friend - and principal investigator of this Golden-cheeked Warbler study - Oliver Komar coauthored an article describing latitudinal sexual segregation for migratory birds wintering in Mexico (Komar et al. 2005). This tendency of sexual segregation on the wintering grounds is of course well-known in many temperate zone breeders that are short to medium distance migrants. Far less is known about birds wintering in the tropics. Using specimen records from 35 different collections, Oliver and his coauthors found significant latitudinal segregation in nine species, six of them parulid warblers. In all but one case - Indigo Bunting being the exception - males were found to winter further north than females. A popular explanation for the phenomenon is that it allows the territorial sex - males - to return earlier to the breeding grounds, to set up territories.

Golden-winged Warblers winter south of Mexico, and thus were not included in that study. Their winter range is from Guatemala south through Central America to northern South America. Honduras forms part of the northern half of the winter range.

Neither the BNA account, nor the Peterson Warbler Guide, nor the Warblers of the Americas guide mention anything about sexual segregation on the wintering grounds for Golden-winged Warbler. I assume this phenomenon has not yet been studied in this particular species. Our ratio of 10:1 appears suggestive.

Literature cited:
BirdLife International (2009) Species factsheet: Vermivora chrysoptera. Downloaded from on 11/3/2010
Confer, John L. 1992. Golden-winged Warbler (Vermivora chrysoptera), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:
Curson, Jon, David Quinn and David Beadle (1994) Warblers of the Americas: An Identification Guide. Houghton Mifflin Company, Boston & New York
Dunn, Jon L. and Kimball Garrett (1997) A field guide to the warblers of North America. Peterson Field Guide Series. Houghton Mifflin Company, Boston & New York
Komar, Oliver, B. J. O'Shea, A. Townsend Peterson and Adolfo G. Navarro-Sigüenza (2005) Evidence of Latitudinal Sexual Segregation among Migratory Birds Wintering in Mexico (Evidencia de la Segregación Latitudinal Sexual en Aves Migratorias durante el Invierno en México). The Auk, Vol. 122, No. 3 (Jul., 2005), pp. 938-94. Stable URL:

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